Optical signal transmitter and optical signal transmission system

ABSTRACT

The invention provides an optical signal transmitter low in noise and in distortion and provides an optical signal transmission system using this optical signal transmitter. The optical signal transmitter includes a plurality of frequency modulation means for distributing an electric signal into a plurality of electric signals and applying frequency modulation to the distributed electric signals to output and a multiplexing mean for multiplexing a plurality of signals output from the plurality of frequency modulation means and outputting a multiplexed signal. The plurality of frequency modulation means are set to be substantially equal to each other in frequency deviation and in intermediate frequency and to be substantially identical to each other in the phase of each output.

TECHNICAL FIELD

The present invention relates to an optical signal transmitter used foroptical transmission of wideband signals, and relates to an opticalsignal transmission system using this optical signal transmitter. Moreparticularly, the present invention relates to an optical signaltransmitter used for optical transmission of multichannel video signalsthat have undergone frequency-division multiplexing and that haveundergone amplitude modulation (abbreviated as “AM”) or quadratureamplitude modulation (abbreviated as “QAM”), and relates to an opticalsignal transmission system using this optical signal transmitter.

BACKGROUND ART

Conventionally, an optical signal transmitter and an optical signaltransmission system employing a method for subjecting video signals,which have undergone frequency-division multiplexing, to frequencymodulation as a single unit (this method will be hereinafter referred toas an “FM batch conversion method”) are known as an optical signaltransmitter and an optical signal transmission system used for opticaltransmission of multichannel video signals that have undergonefrequency-division multiplexing and that have undergone amplitudemodulation or quadrature amplitude modulation.

An optical signal transmitter and an optical signal transmission systemthat employ this FM batch conversion method are disclosed in Non-patentDocument 1.

FIG. 1 shows a structure of a conventional optical signal transmitterand a conventional optical signal transmission system that employ the FMbatch conversion method. FIGS. 2A, 2B, and 2C show signal forms at point“A,” point “B,” and point “C” of FIG. 1, respectively. The opticalsignal transmission system of FIG. 1 is comprises an optical signaltransmitter 80 including an FM batch conversion circuit 81, a lightsource 82, and an optical amplification circuit 83, an opticaltransmission path 85, an optical signal receiver 90 including aphotoelectric conversion circuit 91 and an FM demodulation circuit 92, aset-top box 93, and a television receiver 94. Signal spectra at point“A,” point “B,” and point “C” of FIG. 1 are shown in FIGS. 2A, 2B, and2C, respectively. The same applies to point “A,” point “B,” and point“C” of each figure shown below.

In the optical signal transmitter 80 of FIG. 1, frequency-multiplexedvideo signals shown in FIG. 2A are converted into one widebandfrequency-modulated signal shown in FIG. 2B by the FM batch conversioncircuit 81. The frequency-modulated signal is subjected to intensitymodulation by the light source 82, and is further subjected to opticalamplification by the optical amplification circuit 83, and istransmitted to the optical transmission path 85. In the optical signalreceiver 90, the frequency-modulated signal that has undergone intensitymodulation is photoelectrically converted by the photoelectricconversion circuit 91, and is returned to an electric signal. Thiselectric signal, which is a wideband frequency-modulated signal, issubjected to frequency demodulation by the FM demodulation circuit 92,and the frequency-multiplexed video signals are demodulated as shown inFIG. 2C. The demodulated video signals pass through the set-top box 93,and reach the television receiver 94, whereby a desired video channel isselected.

FIG. 3 shows the structure of an FM batch conversion circuit that isapplicable to the FM batch conversion method (see Patent Document 1,Non-patent Document 2, Non-patent Document 3, for example). The FM batchconversion circuit shown in FIG. 3 uses an optical frequency modulationportion and an optical frequency local oscillation portion. The FM batchconversion circuit 81 comprises the optical frequency modulation portion71, the optical frequency local oscillation portion 72, an opticalmultiplexer 73, and a photodiode 74.

When frequency modulation is performed with a frequency fs by use of acarrier light source having an optical frequency fo in the opticalfrequency modulation portion 71 of the FM batch conversion circuit 81,an optical frequency Ffmld of an optical signal in the output of theoptical frequency modulation portion 71 is expressed as in the followingequation:Ffmld=fo+δf·sin(2π·fs·t)  (1)where δf is a frequency deviation. A DFB-LD (Distributed Feed-Back LaserDiode) is used as the carrier light source of the optical frequencymodulation portion 71.

In the optical frequency local oscillation portion 72, oscillation isperformed by use of an oscillation light source having an opticalfrequency f1. An optical signal transmitted from the local oscillationportion 72 and an optical signal transmitted from the optical frequencymodulation portion 71 are multiplexed by the optical multiplexer 73. TheDFB-LD is used as the oscillation light source of the optical frequencylocal oscillation portion 72. The two optical signals multiplexed by theoptical multiplexer 73 are detected by the photodiode 74 that is anoptical heterodyne detector. The frequency f of the electric signaldetected thereby is expressed as follows:f=fo−f 1 +δf·sin(2π·fs·t)  (2)Herein, if the optical frequency of the carrier light source of theoptical frequency modulation portion 71 and the optical frequency of theoscillation light source of the optical frequency local oscillationportion 72 are caused to come close to each other, it is possible toobtain an electric signal whose frequency is modulated to have anintermediate frequency fi=fo−f1 of several GHz and have a frequencydeviation δf as shown in FIG. 2B.

Generally, the modulation by an input electric current allows the DFB-LDto have an optical frequency varied in the range of several GHz inaccordance with the input electric current, and hence a value of severalGHz can be obtained as the frequency deviation δf. For example, amultichannel AM video signal or QAM video signal that have undergonefrequency multiplication so as to have a frequency range of about 90 MHzto about 750 MHz can be converted by the FM batch conversion circuitinto a frequency-modulated signal having a frequency band of about 6 GHzin which the intermediate frequency fi=fo−f1 becomes equal to about 3GHz as shown in FIG. 2B.

FIG. 4 shows the structure of an FM demodulation circuit applicable tothe optical signal receiver 90. The FM demodulation circuit 92 shown inFIG. 4 is an FM demodulation circuit by delay-line detection, andcomprises a limiter amplifier 76, a delay line 77, an AND gate 78, and alow-pass filter 79.

In the FM demodulation circuit 92, a frequency-modulated optical signalthat has been input is shaped into a square wave by the limiteramplifier 76. The output of the limiter amplifier 76 is branched intotwo output parts, one of which is input to an input terminal of the ANDgate 78 and the other of which undergoes a polarity reversal, is thendelayed by time t by means of the delay line 77, and is input to aninput terminal of the AND gate 78. The output of the AND gate 78 issmoothed by the low-pass filter 79, and is turned intofrequency-demodulated output (see Non-patent Document 1, for example).

A double-tuned frequency discriminator having a resonance circuit, aFoster-Seeley frequency discriminator, and a ratio detection type FMdemodulator can be mentioned as a circuit form of the FM demodulationcircuit, in addition to the FM demodulation circuit by delay-linedetection described here.

Patent Document 1: Japanese Patent No. 2700622;

-   -   Non-patent Document 1: international standard, ITU-T J. 185,        “Transmission equipment for transferring multi-channel        television signals over optical access networks by FM        conversion;”    -   Non-patent Document 2: Shibata et al. “Optical image        distribution system using an FM batch conversion method,”        Institute of Electronics, Information and Communication        Engineers, Technical Journal B, Vol. J83-B, No. 7, July, 2000,        pp. 948-959;    -   Non-patent Document 3: Suzuki et al. “Pulsed FM batch conversion        modulation analog optical CATV distribution method” Institute of        Electronics, Information and Communication Engineers, Autumn        Conference, B-603, 1991.

DISCLOSURE OF THE INVENTION

A low noise and a low distortion are required in transmission of themultichannel video signals described above. According to “Optical imagedistribution system using an FM batch conversion method” by Shibata etal., a CNR (Carrier-to-Noise Ratio) is set to be 42 dB or more, and aCSO (Composite Second-Order Distortion) and a CTB (Composite TripleBeat) are set to be −54 dB or less in an optical signal transmitter andan optical signal transmission system using an FM batch conversionmethod.

However, in the optical signal transmitter using the conventional FMbatch conversion method, the CNR value is in a saturated state between43 dB to 47 dB. Likewise, the CSO value and the CTB value are in asaturated state having a value slightly below −54 dB. If the opticalsignal transmitter can be constructed to have an even lower noise, theCNR can be enlarged, and, as a result, the minimum electric power of theoptical signal receiver whose CNR is 42 dB or more can be reduced. Ifthe minimum light-receiving power of the optical signal receiver can bereduced, the transmission distance can be lengthened, and the opticalbranching ratio can be enlarged.

The DFB-LD of the optical frequency modulation portion used in theconventional FM batch conversion circuit proves difficult inmodification of its design when returning to its structure, and it wasdifficult to realize low-noise characteristics and low-distortioncharacteristics. It is therefore an object of the present invention toprovide an optical signal transmitter low in noise and in distortion andprovide an optical signal transmission system using this optical signaltransmitter.

In order to achieve this object, according to a first aspect of thepresent invention, the present invention is characterized in that anoptical signal transmitter for applying frequency modulation toamplitude-modulated electric signals that have undergone frequencydivision multiplexing to optically transmit the electric signals, theoptical signal transmitter comprising: a distribution circuit fordistributing the electric signals into a plurality of signal parts andoutputting the signal parts; a plurality of frequency modulation meansfor applying frequency modulation to each output of the distributioncircuit and emitting each output, the plurality of frequency modulationmeans being substantially equal to each other in frequency deviation andin intermediate frequency and being substantially identical in the phaseof each output; a multiplexing means for multiplexing outputs of theplurality of frequency modulation means and outputting multiplexedoutputs; and a transmitting circuit for outputting optical signalssubjected to intensity modulation by the output of the multiplexingmeans to an optical transmission path. Herein, the electric signals thathave undergone frequency-division multiplexing and amplitude modulationinclude electric signals that have undergone frequency-divisionmultiplexing and quadrature amplitude modulation.

According to a second aspect of the present invention, the presentinvention is characterized in that an optical signal transmission systemcomprises the optical signal transmitter according to the first aspectof the present invention, a photoelectric conversion means connected tothe optical signal transmitter through an optical transmission path, andan optical signal receiver having a frequency demodulation means forapplying frequency demodulation to an output of the photoelectricconversion means.

The optical signal transmitter and the optical signal transmissionsystem according to the present invention can obtain lower noisecharacteristics and lower distortion characteristics than a conventionaloptical signal transmitter while using conventional electric circuitsand conventional optical circuit components without changing circuitconstants returning to the circuit design of such electric circuits andoptical circuit components.

The low noise characteristics of the optical signal transmitter make itpossible to reduce the minimum light-receiving electric power of theoptical signal receiver, thus making it possible to lengthen thetransmission distance and to enlarge the optical branching ratio betweenthe optical signal transmitter and the optical signal receiver.

Additionally, the low distortion characteristics thereof make itpossible to improve a video-signal receiving quality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a structure of a conventional opticalsignal transmitter and a conventional optical signal transmission systemusing an FM batch conversion method;

FIG. 2A is a view showing signal forms in the optical signal transmitterand the optical signal transmission system;

FIG. 2B is a view showing a signal form in the optical signaltransmitter and the optical signal transmission system;

FIG. 2C is a view showing signal forms in the optical signal transmitterand the optical signal transmission system;

FIG. 3 is a block diagram showing a structure of a conventional FM batchconversion circuit applicable to the FM batch conversion method;

FIG. 4 is a block diagram showing a structure of an FM demodulationcircuit applicable to an optical signal receiver;

FIG. 5 is a block diagram showing a structure of an optical signaltransmitter in which N FM batch conversion circuits, to which electricsignals distributed by a distribution circuit are input while beingmodulated, are used;

FIG. 6 is a block diagram showing a structure of an FM batch conversioncircuit that is applied to an optical signal transmitter and that usesan optical frequency modulation portion;

FIG. 7 is a block diagram showing a structure of an FM batch conversioncircuit that is applied to an optical signal transmitter and that usestwo optical frequency modulation portions for a push-pull structure;

FIG. 8 is a block diagram showing a structure of an FM batch conversioncircuit that is applied to an optical signal transmitter and that uses avoltage-controlled oscillator;

FIG. 9 is a block diagram showing a structure of an FM batch conversioncircuit that is applied to an optical signal transmitter and that usestwo voltage-controlled oscillators for a push-pull structure;

FIG. 10 is a block diagram showing a structure of an optical signaltransmitter including two sets of an optical frequency modulationportion and optical frequency local oscillation portion;

FIG. 11 is a block diagram showing a structure of an optical signaltransmitter in which two optical frequency modulation portions are usedfor a push-pull structure and in which two push-pull structures areused;

FIG. 12 is a block diagram showing a structure of an optical signaltransmitter in which N optical frequency modulation multiplexingcircuits, to which electric signals distributed by a distributioncircuit are input while being modulated, are used;

FIG. 13 is a block diagram showing a structure of the optical frequencymodulation multiplexing circuit;

FIG. 14 is a block diagram showing a structure of an optical signaltransmitter in which N differential optical frequency modulationmultiplexing circuits, to which electric signals distributed by adistribution circuit are input while being modulated, are used; and

FIG. 15 is a block diagram showing a structure of the differentialoptical frequency modulation multiplexing circuit.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be hereinafter described withreference to the accompanying drawings.

A first embodiment of the present invention is an optical signaltransmitter in which N FM batch conversion circuits, to which electricsignals distributed by a distribution circuit are input while beingmodulated, are used, and is an optical signal transmission system usingthis optical signal transmitter. This embodiment of the presentinvention is shown in FIG. 5. FIG. 5 shows a case in which N=3. In FIG.5, the optical signal transmitter 10 includes a distribution circuit 11,FM batch conversion circuits 12, an optical multiplexing circuit 13, alight source 14 serving as a transmitting circuit, an opticalamplification circuit 15, and an optical transmission path 85. The lightsource 14 may include a semiconductor laser and a drive circuit thatdrives this semiconductor laser as a transmitting circuit, and thetransmitting circuit may include the optical amplification circuit 15.

In FIG. 5, when multichannel AM video signals or QAM video signals thathave undergone frequency multiplication in the frequency range of about90 MHz to about 750 MHz as shown in FIG. 2A are input to the opticalsignal transmitter 10, the signals are distributed into three groups ofsignals by the distribution circuit 11. Each output of the distributioncircuit 11 is input to the FM batch conversion circuits 12 as amodulated input, and is subjected to frequency modulation by the FMbatch conversion circuits 12. Outputs of the three FM batch conversioncircuits 12 are multiplexed by the optical multiplexing circuit 13. Theoutput of the optical multiplexing circuit 13 is a widebandfrequency-modulated electric signal as shown in FIG. 2B. Thisfrequency-modulated electric signal is converted into an optical signalsubjected to intensity modulation by the light source 14. The opticalsignal is amplified to a predetermined optical level by the opticalamplification circuit 15, and is transmitted to the optical transmissionpath 85.

Herein, if the three FM batch conversion circuits 12 are set to be equalto each other in frequency deviation and in intermediate frequency andare set to be identical to each other in the phase of each output of FMbatch conversion circuits, the electric signals multiplexed by theoptical multiplexing circuit 13 have their noise quantities expressed asthe sum total of electric powers of the three FM batch conversioncircuits 12, i.e., as an electric-power addition, and have their signalcomponents expressed as the sum total of voltages thereof, i.e., as avoltage addition. Since the three FM batch conversion circuits 12 areset to be identical to each other in the phase of each output, it ispossible to, for example, adjust the length of a transmission path, suchas an optical fiber, or use a phase adjuster.

Let the voltages of signal components output from the three FM batchconversion circuits 12 be designated as Vs1, Vs2, and Vs3, respectively,and let Vs1=Vs2=Vs3=Vs. In this case, the sum total Vst of the voltagesof signal components output from the optical multiplexing circuit 13 areexpressed as follows:Vst=Vs 1 +Vs 2 +Vs 3=3Vs  (3)

Under the condition that the output of only one of the three FM batchconversion circuits 12 is input to the optical multiplexing circuit 13,the signal power Ps1 of the output of the optical multiplexing circuit13 is expressed as follows:Ps 1 =Vs ² /R  (4)where R is an input impedance of the light source 14. Under thecondition that the outputs of the three FM batch conversion circuits 12are input to the optical multiplexing circuit 13, the signal power Pstof the output of the optical multiplexing circuit 13 is expressed asfollows:Pst=(Vst)² /R=9Vs ² /R  (5)Therefore, the electric-power ratio between the signal power Ps1 and thesignal power Ps3 is expressed as follows:10log(Pst/Ps 1)=20log(3)[dB]  (6)

On the other hand, let the electric powers of noise components outputfrom the three FM batch conversion circuits 12 be designated as Pn1,Pn2, and Pn3, respectively, and let Pn1=Pn2=Pn3=Pn. Since anelectric-power addition is applied to noise components, the sum totalPnt of the electric powers of noise components output from the opticalmultiplexing circuit 13 is expressed as follows:Pnt=Pn 1 +Pn 2 +Pn 3=3Pn  (7)If the output of only one of the three FM batch conversion circuits 12is input to the optical multiplexing circuit 13, the noise power Pn1output from the optical multiplexing circuit 13 is expressed as follows:Pn1=Pn  (8)Therefore, the electric-power ratio between the noise power Pn1 and thenoise power Pnt is expressed as follows:10log(Pnt/Pn 1)=10log(3)[dB]  (9)

From this fact, it is understood that, in a case in which the three FMbatch conversion circuits are used, the signal power ratio becomes equalto 20log(3)[dB], but the noise power ratio becomes equal to10log(3)[dB], and hence the signal-to-noise power in the output of theoptical multiplexing circuit is improved by 10log(3)[dB] in comparisonwith a case in which only one of the three FM batch conversion circuitsis used. Although the structure using the three FM batch conversioncircuits is shown in the embodiment of FIG. 5, the signal-to-noise powercan be improved by using two or more FM batch conversion circuits. In acase in which N FM batch conversion circuits are used (N is an integerwhich is two or greater), the signal-to-noise power can be improved by10log(N)[dB] in comparison with a case in which only one FM batchconversion circuit is used.

With regard to distortions, the three FM batch conversion circuits aredifferent from each other in distortion characteristics, and, if theyhave distortion characteristics opposite in direction, offsetting can beachieved in proportion to opposite distortions by a wave combination,and hence the distortions can be made lower than a case in which onlyone FM batch conversion circuit is used.

If the optical signal transmitter 10 of FIG. 5, instead of the opticaltransmitter 80, is applied to the optical signal transmission system inFIG. 1, the minimum light-receiving electric power of the optical signalreceiver can be reduced, and the transmission distance can belengthened, and the optical branching ratio can be enlarged between theoptical signal transmitter and the optical signal receiver.Additionally, if low distortion characteristics can be realized by theoptical signal transmitter, the quality of receiving video signals canbe improved.

Although the signal of FIG. 2A is used as an example of a signal to beinput to the optical signal transmitter in this embodiment, theinvention is not limited to this signal form.

Next, a second embodiment of the present invention is a structure of anFM batch conversion circuit that is applied to the optical signaltransmitter described in the first embodiment and that uses an opticalfrequency modulation portion. This embodiment of the present inventionis shown in FIG. 6. In FIG. 6, the FM batch conversion circuit 12includes an optical frequency modulation portion 22, an opticalfrequency local oscillation portion 32, an optical multiplexing portion23, and an optical detector 24.

In the FM batch conversion circuit 12, when the frequency-multiplexedvideo signals shown in FIG. 2A are subjected to frequency modulation byuse of the carrier light source having an optical frequency fo in theoptical frequency modulation portion 22, the optical frequency Ffmld ofan optical signal in the output of the optical frequency modulationportion 22 is calculated from Equation (1) mentioned above where δf is afrequency deviation. In Equation (1), the modulated signal is a signalhaving a frequency fs. A DFB-LD (Distributed Feed-Back Laser Diode) canbe used as the carrier light source of the optical frequency modulationportion 22.

In the optical frequency local oscillation portion 32, oscillation isperformed by use of an oscillating light source having an opticalfrequency f1, and the signal is multiplexed with an optical signalemitted from the optical frequency modulation portion 22 by the opticalmultiplexer 23. The DFB-LD can be used as the oscillating light sourceof the optical frequency local oscillation portion 32. The two opticalsignals multiplexed by the optical multiplexer 23 are subjected toheterodyne detection by the optical detector 23. A photodiode thatfunctions as a heterodyne detector can be used as the optical detector.The frequency f of the electric signal subjected to heterodyne detectionby the optical detector 24 is calculated from Equation (2) mentionedabove. In Equation (2), the modulated signal is a signal having afrequency fs. Herein, if the optical frequency of the carrier lightsource of the optical frequency modulation portion 22 and the opticalfrequency of the oscillating light source of the local oscillationportion 32 are caused to come close to each other, it is possible toobtain an electric signal in which frequency is modulated to have anintermediate frequency fi=fo−f1 of several GHz and have a frequencydeviation γf as shown in FIG. 2B.

Generally, the modulation by an input electric current allows the DFB-LDto have an optical frequency varied in the range of several GHz inaccordance with the input electric current, and hence a value of severalGHz can be obtained as the frequency deviation γf. For example,multichannel AM video signals or QAM video signals that have undergonefrequency multiplication so as to have a frequency range of about 90 MHzto about 750 MHz can be converted by the FM batch conversion circuitinto a frequency-modulated signal having a frequency band of about 6 GHzin which the intermediate frequency fi=fo−f1 becomes equal to about 3GHz as shown in FIG. 2B.

Further, each intermediate frequency fi, which is a frequency equal to adifference between the optical frequency of the carrier light source ofthe optical frequency modulation portion 22 and the optical frequency ofthe oscillating light source of the optical frequency local oscillationportion 32 used in N FM batch conversion circuits, is set to besubstantially equal in the N FM batch conversion circuits, and frequencymodulation is performed with substantially the same frequency deviationcentering on this intermediate frequency. Further, the N FM batchconversion circuits are set to be substantially identical to each otherin the phase of each output. Thus, the output of the opticalmultiplexing circuit 13 of FIG. 5 has its noise quantity expressed asthe sum total of electric powers, i.e., as an electric-power additionand has its signal component expressed as the sum total of voltages,i.e., as a voltage addition. For example, the length of a transmissionpath, such as an optical fiber, can be adjusted, or a phase adjuster canbe used, in order to set them so that the phase of each output becomesmutually identical.

From this fact, it is understood that, when use is made of an opticalsignal transmitter that uses N sets of optical frequency modulationportions and optical frequency local oscillation portions, the signalpower ratio becomes 20log(N)[dB], but the noise power ratio becomes10log(N)[dB], and hence the signal-to-noise power in the output of theoptical multiplexing circuit 13 of FIG. 5 is improved by 10log(N)[dB] incomparison with a case in which use is made of an optical signaltransmitter that uses one set of an optical frequency modulation portionand an optical frequency local oscillation portion.

With regard to distortions, the N sets of optical frequency modulationportions are different from each other in distortion characteristics,and, if they have distortion characteristics opposite in direction,offsetting can be achieved in proportion to opposite distortions by awave combination, and hence the distortions can be made lower than acase in which only one FM batch conversion circuit is used.

If the thus formed N FM batch conversion circuits are applied to anoptical signal transmitter, the minimum light-receiving electric powerof an optical signal receiver in an optical signal transmission systemcan be reduced, and the transmission distance can be lengthened, and theoptical branching ratio can be enlarged between the optical signaltransmitter and the optical signal receiver. Additionally, if lowdistortion characteristics can be obtained by the optical signaltransmitter, the quality of receiving video signals can be improved.

Although the signal of FIG. 2A is used as an example of a signal to beinput to the optical signal transmitter in this embodiment, theinvention is not limited to this signal form.

Next, a third embodiment of the present invention is a structure of anFM batch conversion circuit that is applied to the optical signaltransmitter described in the first embodiment and that uses two opticalfrequency modulation portions for a push-pull structure. This embodimentof the present invention is shown in FIG. 7. In FIG. 7, the FM batchconversion circuit 12 includes a differential distributor 21, an opticalfrequency modulation portion 22-1, an optical frequency modulationportion 22-2, an optical multiplexser 23, and an optical detector 24.

In the FM batch conversion circuit 12, a frequency-multiplexed videosignal, such as that shown in FIG. 2A, is distributed by thedifferential distributor 21 into two electric signals whose phases havebeen inverted. If one of the two electric signals distributed by thedifferential distributor 21 is a modulated input, and if frequencymodulation is performed by use of a carrier light source having anoptical frequency fo1 in the optical frequency modulation portion 22-1,the optical frequency Ffmld1 of an optical signal in the output of theoptical frequency modulation portion 22-1 is expressed as follows:Ffmld 1 =fo 1+(δf/2)·sin(2π·fs·t)  (10)where δf/2 is a frequency deviation. In Equation (10), the modulatedsignal is a signal having a frequency fs. If the other one of the twoelectric signals distributed by the differential distributor is amodulated input, and if frequency modulation is performed by use of acarrier light source having an optical frequency fo2 in the opticalfrequency modulation portion 22-2, the optical frequency Ffmld2 of anoptical signal in the output of the optical frequency modulation portion22-2 is expressed as follows:Ffmld 2 =fo 2−(δf/2)·sin(2π·fs·t)  (11)where δf/2 is a frequency deviation. In Equation (11), the modulatedsignal is a signal having a frequency fs. ADFB-LD (Distributed Feed-BackLaser Diode) can be used as a carrier light source for the opticalfrequency modulation portions 22-1 and 22-2.

Outputs emitted from the optical frequency modulation portions 22-1 and22-2 are multiplexed by the optical multiplexer 23, and the two opticalsignals multiplexed by the optical multiplexer 23 are subjected toheterodyne detection by the optical detector 23. A photodiode thatfunctions as a heterodyne detector can be used as the optical detector.The frequency f of the electric signal subjected to heterodyne detectionby the optical detector 24 is expressed as a frequency equal to adifference between the values shown in Equations (10) and (11) asfollows:f=fo 1 −fo 2 +δf·sin(2π·fs·t)  (12)In Equation (12), the modulated signal is a signal having a frequencyfs. Herein, if the optical frequency of the carrier light source of theoptical frequency modulation portion 22-1 and the optical frequency ofthe carrier light source of the optical frequency modulation portion22-2 are caused to come close to each other, it is possible to obtain anelectric signal in which frequency is modulated to have an intermediatefrequency fi=fo−f1 of several GHz and have a frequency deviation δf asshown in FIG. 2B.

Generally, the modulation by an input electric current allows the DFB-LDto have an optical frequency varied in the range of several GHz inaccordance with the input electric current, and hence a value of severalGHz can be obtained as the frequency deviation δf. For example,multichannel AM video signals or QAM video signals that have undergonefrequency multiplication so as to have a frequency range of about 90 MHzto about 750 MHz can be converted by the FM batch conversion circuitinto a frequency-modulated signal having a frequency band of about 6 GHzin which the intermediate frequency fi=fo−f1 becomes equal to about 3GHz as shown in FIG. 2B.

Further, each intermediate frequency fi, which is a frequency equal to adifference between the optical frequency of the carrier light source ofthe optical frequency modulation portion 22-1 and the optical centerfrequency of the carrier light source of the optical frequencymodulation portion 22-2 used in N FM batch conversion circuits, is setto be substantially equal in the N FM batch conversion circuits, andfrequency modulation is performed with substantially the same frequencydeviation centering on this intermediate frequency. Further, the N FMbatch conversion circuits are set to be substantially identical to eachother in the phase of each output. Thus, the output of the opticalmultiplexing circuit 13 of FIG. 5 has its noise quantity expressed asthe sum total of electric powers, i.e., as an electric-power additionand has its signal component expressed as the sum total of voltages,i.e., as a voltage addition. For example, the length of a transmissionpath, such as an optical fiber, can be adjusted, or a phase adjuster canbe used, in order to set them so that the phase of each output becomesmutually identical.

From this fact, it is understood that, when use is made of an opticalsignal transmitter that uses N sets of optical frequency modulationportions, the signal power ratio becomes 20log(N)[dB], however, thenoise power ratio becomes 10log(N)[dB], and hence the signal-to-noisepower in the output of the optical multiplexing circuit 13 of FIG. 5 isimproved by 10log(N)[dB] in comparison with a case in which use is madeof an optical signal transmitter that uses one set of optical frequencymodulation portions.

With regard to distortions, the N sets of optical frequency modulationportions are different from each other in distortion characteristics,and, if they have distortion characteristics opposite in direction,offsetting can be achieved in proportion to opposite distortions by awave combination, and hence the distortions can be made lower than acase in which only one FM batch conversion circuit is used.

If the thus formed N FM batch conversion circuits are applied to anoptical signal transmitter, the minimum light-receiving electric powerof an optical signal receiver in an optical signal transmission systemcan be reduced, and the transmission distance can be lengthened, and theoptical branching ratio can be enlarged between the optical signaltransmitter and the optical signal receiver. Additionally, if lowdistortion characteristics can be obtained by the optical signaltransmitter, the quality of receiving video signals can be improved.

Although the signal of FIG. 2A is used as an example of a signal to beinput to the optical signal transmitter in this embodiment, theinvention is not limited to this signal form.

Next, a fourth embodiment of the present invention is an FM batchconversion circuit that is applied to the optical signal transmitterdescribed in the first embodiment and that uses a voltage-controlledoscillator. This embodiment of the present invention is shown in FIG. 8.In FIG. 8, the FM batch conversion circuit 12 includes avoltage-controlled oscillator 26.

In the FM batch conversion circuit 12, a frequency-multiplexed videosignal, such as that shown in FIG. 2A, is subjected to frequencymodulation with a frequency fo as the center frequency in thevoltage-controlled oscillator 26, and a frequency fv of an electricsignal that has been output is expressed as follows when the frequencydeviation is δd:fv=fo+δf·sin(2π·fs·t)  (13)Thus, a frequency-modulated signal is obtained which has an intermediatefrequency fi=fo and a frequency deviation δf. In Equation (13), themodulated signal is a signal having a frequency fs.

For example, multichannel AM video signals or QAM video signals thathave undergone frequency multiplication so as to have a frequency rangeof about 90 MHz to about 750 MHz can be converted by the FM batchconversion circuit into a frequency-modulated signal having a frequencyband of about 6 GHz in which the intermediate frequency fi=fo−f1 becomesequal to about 3 GHz as shown in FIG. 2B.

Further, each intermediate frequency fi of the voltage-controlledoscillator 26 used in N FM batch conversion circuits, is set to besubstantially equal in the N FM batch conversion circuits, and frequencymodulation is performed with substantially the same frequency deviationcentering on this intermediate frequency. Further, the N FM batchconversion circuits are set to be substantially identical to each otherin the phase of each output. Thus, the output of the opticalmultiplexing circuit 13 of FIG. 5 has its noise quantity expressed asthe sum total of electric powers, i.e., as an electric-power additionand has its signal component expressed as the sum total of voltages,i.e., as a voltage addition. For example, the length of a transmissionpath, such as an optical fiber, can be adjusted, or a phase adjuster canbe used, in order to set them so that the phase of each output becomesmutually identical.

From this fact, it is understood that, when use is made of an opticalsignal transmitter that uses N voltage-controlled oscillators, thesignal power ratio becomes 20log(N)[dB], however, the noise power ratiobecomes 10log(N)[dB], and hence the signal-to-noise power in the outputof the optical multiplexing circuit 13 of FIG. 5 is improved by10log(N)[dB] in comparison with a case in which use is made of anoptical signal transmitter that uses only one voltage-controlledoscillator.

With regard to distortions, the N voltage-controlled oscillators aredifferent from each other in distortion characteristics, and, if theyhave distortion characteristics opposite in direction, offsetting can beachieved in proportion to opposite distortions by a wave combination,and hence the distortions can be made lower than a case in which onlyone FM batch conversion circuit is used.

If the thus formed N FM batch conversion circuits are applied to anoptical signal transmitter, the minimum light-receiving electric powerof an optical signal receiver in an optical signal transmission systemcan be reduced, and the transmission distance can be lengthened, and theoptical branching ratio can be enlarged between the optical signaltransmitter and the optical signal receiver. Additionally, if lowdistortion characteristics can be obtained by the optical signaltransmitter, the quality of receiving video signals can be improved.

Although the signal of FIG. 2A is used as an example of a signal to beinput to the optical signal transmitter in this embodiment, theinvention is not limited to this signal form.

Next, a fifth embodiment of the present invention is an FM batchconversion circuit that is applied to the optical signal transmitterdescribed in the first embodiment and that uses two voltage-controlledoscillators for a push-pull structure. This embodiment of the presentinvention is shown in FIG. 9. In FIG. 9, the FM batch conversion circuit12 includes a differential distributor 21, a voltage-controlledoscillator 28-1, a voltage-controlled oscillator 28-2, a mixer 29, and alow-pass filter 30.

In the FM batch conversion circuit 12, a frequency-multiplexed videosignal, such as that shown in FIG. 2A, is distributed by thedifferential distributor 21 into two electric signals in which phaseshave been inverted. If one of the two electric signals distributed bythe differential distributor 21 is subjected to frequency modulationwith a frequency fo as the center frequency in the voltage-controlledoscillator 28-1, a frequency fv1 of an electric signal output therefromis expressed as follows:fv 1 =fo 1+(δf/2)·sin(2π·fs·t)  (14)where πf/2 is a frequency deviation. Thus, a frequency-modulated signalwhich has an intermediate frequency fi=fo1 and a frequency deviationδf/2 is obtained. In Equation (14), the modulated signal is a signalhaving a frequency fs. If the other one of the two electric signalsdistributed by the differential distributor 21 is a modulated input, andis subjected to frequency modulation with a frequency fo1 as the centerfrequency in the voltage-controlled oscillator 28-2, a frequency fv2 ofan electric signal output therefrom is expressed as follows:fv 2 =fo 2−(δf/2)·sin(2π·fs·t)  (15)where δf/2 is a frequency deviation. Thus, a frequency-modulated signalwhich has an intermediate frequency fi=fo2 and a frequency deviationδf/2 is obtained. In Equation (15), the modulated signal is a signalhaving a frequency fs.

Outputs from the voltage-controlled oscillators 28-1 and 28-2 are mixedtogether by the mixer 29. The two electric signals mixed together by themixer 29 are then smoothed by the low-pass filter 30. The frequency f ofthe electric signal smoothed by the low-pass filter 30 that transmits anelectric signal having a frequency equal to a difference between theintermediate frequency fo1 and the intermediate frequency fo2 isexpressed as that of an electric signal having a frequency equal to adifference between the value of Equation (14) and the value of Equation(15) as follows:f=fo 1 −fo 2 +δf·sin(2π·fs·t)  (16)In Equation (16), the modulated signal is a signal having a frequencyfs. Herein, it is possible to obtain an electric signal whose frequencyis modulated to have an intermediate frequency fi=fo1−fo2 of several GHzand have a frequency deviation δf as shown in FIG. 2B.

For example, multichannel AM video signal or QAM video signal that haveundergone frequency multiplication so as to have a frequency range ofabout 90 MHz to about 750 MHz can be converted by the FM batchconversion circuit into a frequency-modulated signal having a frequencyband of about 6 GHz in which the intermediate frequency fi=fo−f1 becomesequal to about 3 GHz as shown in FIG. 2B.

Further, each intermediate frequency fi, which is a frequency equal to adifference between the voltage-controlled oscillator 28-1 and thevoltage-controlled oscillator 28-2 used in N FM batch conversioncircuits, is set to be substantially equal in the N FM batch conversioncircuits, and frequency modulation is performed with substantially thesame frequency deviation centering on this intermediate frequency.Further, the N FM batch conversion circuits are set to be substantiallyidentical to each other in the phase of each output. Thus, the output ofthe optical multiplexing circuit 13 of FIG. 5 has its noise quantityexpressed as the sum total of electric powers, i.e., as anelectric-power addition and has its signal component expressed as thesum total of voltages, i.e., as a voltage addition. For example, thelength of a transmission path, such as an optical fiber, can beadjusted, or a phase adjuster can be used, in order to set them so thatthe phase of each output becomes mutually identical.

From this fact, it is understood that, when use is made of an opticalsignal transmitter that uses N voltage-controlled oscillators, thesignal power ratio becomes 20log(N)[dB], but the noise power ratiobecomes 10log(N)[dB], and hence the signal-to-noise power in the outputof the optical multiplexing circuit 13 of FIG. 5 is improved by10log(N)[dB] in comparison with a case in which use is made of anoptical signal transmitter that uses only one voltage-controlledoscillator.

With regard to distortions, the N optical frequency modulation portionsare different from each other in distortion characteristics, and, ifthey have distortion characteristics opposite in direction, offsettingcan be achieved in proportion to opposite distortions by a wavecombination, and hence the distortions can be made lower than a case inwhich only one FM batch conversion circuit is used.

If the thus formed N FM batch conversion circuits are applied to anoptical signal transmitter, the minimum light-receiving electric powerof an optical signal receiver in an optical signal transmission systemcan be reduced, and the transmission distance can be lengthened, and theoptical branching ratio can be enlarged between the optical signaltransmitter and the optical signal receiver. Additionally, if lowdistortion characteristics can be obtained by the optical signaltransmitter, the quality of receiving video signals can be improved.

Although the signal of FIG. 2A is used as an example of a signal to beinput to the optical signal transmitter in this embodiment, theinvention is not limited to this signal form.

Next, a sixth embodiment of the present invention is an optical signaltransmitter using two sets of optical frequency modulation portions andoptical frequency local oscillation portions, and is an optical signaltransmission system using this optical signal transmitter. Thisembodiment of the present invention is shown in FIG. 10. In FIG. 10, theoptical signal transmitter 10 includes a distribution circuit 11, anoptical frequency modulation portion 22-1, an optical frequencymodulation portion 22-2, an optical frequency local oscillation portion32-1, an optical frequency local oscillation portion 32-2, an opticalmultiplexer 25-1, an optical multiplexer 25-2, an optical multiplexer27, an optical detector 24, a light source 14 serving as a transmittingcircuit, an optical amplification circuit 15, and an opticaltransmission path 85. The light source 14 may include a semiconductorlaser and a drive circuit, which drives this semiconductor laser,serving as a transmitting circuit. The transmitting circuit may includethe optical amplification circuit 15.

In FIG. 10, when multichannel AM video signals or QAM video signals thathave undergone frequency multiplication so as to have a frequency rangeof about 90 MHz to about 750 MHz as shown in FIG. 2A are input to theoptical signal transmitter 10, the signals are distributed by thedistribution circuit 11 into two signal arts. One output of thedistribution circuit 11 is input to the optical frequency modulationportion 22-1 as a modulated input, and is subjected to frequencymodulation. The other output of the distribution circuit 11 is input tothe optical frequency modulation portion 22-2 as a modulated input, andis subjected to frequency modulation.

An optical signal subjected to frequency modulation by the opticalfrequency modulation portion 22-1 is multiplexed with local oscillationlight emitted from the local oscillation portion 32-1 by the opticalmultiplexer 25-1 while being caused to have the same polarizationdirection. Herein, the optical frequency of the optical frequency localoscillation portion 32-1 is apart from the optical center frequency ofthe frequency-modulated optical signal output from the optical frequencymodulation portion 22-1 by a frequency substantially equal to theintermediate frequency.

An optical signal subjected to frequency modulation by the opticalfrequency modulation portion 22-2 is multiplexed with local oscillationlight emitted from the local oscillation portion 32-2 by the opticalmultiplexer 25-2 while being caused to have the same polarizationdirection. Herein, the optical frequency of the optical frequency localoscillation portion 32-2 is apart from the optical center frequency ofthe frequency-modulated optical signal output from the optical frequencymodulation portion 22-2 by a frequency substantially equal to theintermediate frequency.

The optical signals output from the optical multiplexers 25-1 and 25-2,respectively, are multiplexed by the optical multiplexer 27 while makingthe polarization direction of the optical signal output from the opticalmultiplexer 25-1 perpendicular to the polarization direction of thesecond optical signal output from the optical multiplexer 25-2, and amultiplexed signal is output therefrom. The optical signal output fromthe optical multiplexer 27 is subjected to heterodyne detection in theoptical detector 24, and an electric signal, which has a frequency equalto a difference between the optical frequency of the optical signalemitted from the optical frequency modulation portion and the opticalfrequency of the local oscillation light emitted from the localoscillation portion, is output. A photodiode that performs heterodynedetection can be used as the detector 24. The output of this detector 24is a wideband frequency-modulated electric signal as shown in FIG. 2B.This frequency-modulated electric signal is converted into an opticalsignal subjected to intensity modulation by the light source 14, is thenamplified to a predetermined optical level by the optical amplificationcircuit 15, and is transmitted to the optical transmission path 85. Asemiconductor laser, such as a DFB-LD, can be used as the light source.

Herein, the frequency deviations of the two optical frequency modulationportions 22-1 and 22-2 are set to be substantially equal to each other.Further, a difference between the optical frequency of the opticalsignal of the optical frequency modulation portion 22-1 and the opticalfrequency of the local oscillation light of the local oscillationportion 32-1 is set to be substantially equal to a difference betweenthe optical frequency of the optical signal of the optical frequencymodulation portion 22-2 and the optical frequency of the localoscillation light of the local oscillation portion 32-2. Further, thephase of an electric signal obtained by subjecting the multiplexedoptical signal emitted from the optical multiplexer 25-1 to heterodynedetection by the optical detector 24 is set to be substantially equal tothe phase of an electric signal obtained by subjecting the multiplexedoptical signal emitted from the optical multiplexer 25-2 to heterodynedetection by the optical detector 24. Thereby, the electric signaldetected by the optical detector 24 has its noise quantity expressed asthe sum total of electric powers, i.e., as an electric-power additionand has its signal component expressed as the sum total of voltages,i.e., as a voltage addition. For example, the length of a transmissionpath, such as an optical fiber, can be adjusted, or a phase adjuster canbe used, in order to set them so that the phase of each output becomesmutually identical.

From this fact, it is understood that, when use is made of an opticalsignal transmitter that uses two sets of optical frequency modulationportions and optical frequency local oscillation portions, the signalpower ratio becomes 20log(2)[dB], however, the noise power ratio becomes10log(2)[dB], and hence the signal-to-noise power in the output of theoptical multiplexing circuit is improved by 10log(2)[dB] in comparisonwith a case in which use is made of an optical signal transmitter thatuses only one set of an optical frequency modulation portion and anoptical frequency local oscillation portion.

With regard to distortions, the two sets of optical frequency modulationportions are different from each other in distortion characteristics,and, if they have distortion characteristics opposite in direction,offsetting can be achieved in proportion to opposite distortions by awave combination, and hence the distortions can be made lower than acase in which only one FM batch conversion circuit is used.

If the optical signal transmitter 10 of FIG. 10, instead of the opticaltransmitter 80, is applied to the optical signal transmission system inFIG. 1, the minimum light-receiving electric power of the optical signalreceiver can be reduced, and the transmission distance can belengthened, and the optical branching ratio can be enlarged between theoptical signal transmitter and the optical signal receiver.

Additionally, if low distortion characteristics can be realized by theoptical signal transmitter, the quality of receiving video signals canbe improved.

Although the signal of FIG. 2A is used as an example of a signal to beinput to the optical signal transmitter in this embodiment, theinvention is not limited to this signal form.

Next, a seventh embodiment of the present invention is an optical signaltransmitter including two sets of two push-pull structure opticalfrequency modulation portions therein, and is an optical signaltransmission system using this optical signal transmitter. Thisembodiment of the present invention is shown in FIG. 11. In FIG. 11, theoptical signal transmitter 10 comprises a distribution circuit 11, adifferential distributor 21-1, a differential distributor 21-2, anoptical frequency modulation portion 22-1, an optical frequencymodulation portion 22-2, an optical frequency modulation portion 22-3,an optical frequency modulation portion 22-4, an optical multiplexer25-1, an optical multiplexer 25-2, an optical multiplexer 27, an opticaldetector 24, a light source 14 serving as a transmitting circuit, anoptical amplification circuit 15, and an optical transmission path 85.The light source 14 may include a semiconductor laser and a drivecircuit that drives this semiconductor laser as a transmitting circuit,and the transmitting circuit may include the optical amplificationcircuit 15.

In FIG. 11, when multichannel AM video signals or QAM video signals thathave undergone frequency multiplication so as to have a frequency rangeof about 90 MHz to about 750 MHz as shown in FIG. 2A are input to theoptical signal transmitter 10, the signals are distributed by thedistribution circuit 11 into two signal parts. One output of thedistribution circuit 11 is distributed by the differential distributor21-1 into two electric signals in which phases have been inverted. Theoptical frequency Ffmld1 of the output light emitted from the opticalfrequency modulation portion 22-1 is subjected to frequency modulationby one of the two electric signals emitted from the differentialdistributor 21-1, and thereby a frequency-modulated optical signal isoutput. The optical frequency Ffmld2 of the output light emitted fromthe optical frequency modulation portion 22-2 is subjected to frequencymodulation by the other one of the two electric signals emitted from thedifferential distributor 21-1, and thereby a frequency-modulated opticalsignal is output. The frequency-modulated optical signal emitted fromthe optical frequency modulation portion 22-1 and thefrequency-modulated optical signal emitted from the optical frequencymodulation portion 22-2 are set so that a difference in the opticalcenter frequency becomes substantially equal to an intermediatefrequency and so that coincidence of the polarization direction isachieved, are then multiplexed by the optical multiplexer 25-1, and areturned into a first optical signal.

The other output of the distribution circuit 11 is distributed by thedifferential distributor 21-2 into two electric signals in which phaseshave been inverted. The optical frequency Ffmld3 of the output lightemitted from the optical frequency modulation portion 22-3 is subjectedto frequency modulation by one of the two electric signals emitted fromthe differential distributor 21-2, and thereby a frequency-modulatedoptical signal is output. The optical frequency Ffmld4 of the outputlight emitted from the optical frequency modulation portion 22-2 issubjected to frequency modulation by the other one of the two electricsignals emitted from the differential distributor 21-4, and thereby afrequency-modulated optical signal is output. The frequency-modulatedoptical signal emitted from the optical frequency modulation portion22-3 and the frequency-modulated optical signal emitted from the opticalfrequency modulation portion 22-4 are set so that a difference in theoptical center frequency becomes substantially equal to an intermediatefrequency and so that coincidence of the polarization direction isachieved, are then multiplexed by the optical multiplexer 25-2, and areturned into a second optical signal.

The first optical signal output from the optical multiplexers 25-1 andthe second optical signal output from the optical multiplexers 25-2, aremultiplexed by the optical multiplexer 27 while making the polarizationdirection of the first optical signal perpendicular to the polarizationdirection of the second optical signal respectively, and a multiplexedsignal is output. The optical signal output from the optical multiplexer27 is subjected to heterodyne detection in the optical detector 24, andan electric signal is output. This electric signal has a frequency equalto a difference between the optical frequency of the frequency-modulatedoptical signal emitted from the optical frequency modulation portion22-1 and the optical frequency of the frequency-modulated optical signalemitted from the optical frequency modulation portion 22-2 and equal toa difference between the optical frequency of the frequency-modulatedoptical signal emitted from the modulation portion optical frequency22-3 and the optical frequency of the frequency-modulated optical signalemitted from the optical frequency modulation portion 22-4. A photodiodethat performs heterodyne detection can be used as the detector 24. Theoutput of this detector 24 is a wideband frequency-modulated electricsignal as shown in FIG. 2B. This frequency-modulated electric signal isconverted into an optical signal subjected to intensity modulation bythe light source 14, is then amplified to a predetermined optical levelby the optical amplification circuit 15, and is transmitted to theoptical transmission path 85. A semiconductor laser, such as a DFB-LD,can be used as the light source.

Herein, the frequency deviations of the optical frequency modulationportion 22-1, the optical frequency modulation portion 22-2 the opticalfrequency modulation portion 22-3, and the optical frequency modulationportion 22-4 are set to be substantially equal to each other. Also, thephase of an electric signal obtained by subjecting the multiplexedoptical signal emitted from the optical multiplexer 25-1 to heterodynedetection by the optical detector 24 is set to be substantially equal tothe phase of an electric signal obtained by subjecting the multiplexedoptical signal emitted from the optical multiplexer 25-2 to heterodynedetection by the optical detector 24. Thereby, the electric signaldetected by the optical detector 24 has its noise quantity expressed asthe sum total of electric powers, i.e., as an electric-power additionand has its signal component expressed as the sum total of voltages,i.e., as a voltage addition. For example, the length of a transmissionpath, such as an optical fiber, can be adjusted, or a phase adjuster canbe used, in order to set them so that the phase of each output becomesmutually identical.

From this fact, it is understood that, when use is made of an opticalsignal transmitter that uses two sets of optical frequency modulationportions, in which each set consisting of two optical frequencymodulation portions serves as a push-pull structure, the signal powerratio becomes 20log(2)[dB], however, the noise power ratio becomes10log(2)[dB], and hence the signal-to-noise power in the output of theoptical multiplexing circuit is improved by 10log(2)[dB] in comparisonwith a case in which use is made of an optical signal transmitter thatuses only one set of two optical frequency modulation portions servingas a push-pull structure.

With regard to distortions, the two optical frequency modulationportions serving as a push-pull structure are different from each otherin distortion characteristics, and, if they have distortioncharacteristics opposite in direction, offsetting can be achieved inproportion to opposite distortions by a wave combination, and hence thedistortions can be made lower than a case in which only one FM batchconversion circuit is used.

If the optical signal transmitter 10 of FIG. 11, instead of the opticaltransmitter 80, is applied to the optical signal transmission system inFIG. 1, the minimum light-receiving electric power of the optical signalreceiver can be reduced, and the transmission distance can belengthened, and the optical branching ratio can be enlarged between theoptical signal transmitter and the optical signal receiver.

Additionally, if low distortion characteristics can be realized by theoptical signal transmitter, the quality of receiving video signals canbe improved.

Although the signal of FIG. 2A is used as an example of a signal to beinput to the optical signal transmitter in this embodiment, theinvention is not limited to this signal form.

Next, an eighth embodiment of the present invention is an optical signaltransmitter that uses N optical frequency modulation multiplexingcircuits to which electric signals distributed by a distribution circuitare input while being modulated, and is an optical signal transmissionsystem using this optical signal transmitter. This embodiment of thepresent invention is shown in FIG. 12. In FIG. 12, the optical signaltransmitter 10 comprises a distribution circuit 11, an optical frequencymodulation multiplexing circuit 33, an optical multiplexing circuit 34,an optical detection circuit 35, alight source 14 serving as atransmitting circuit, an optical amplification circuit 15, and anoptical transmission path 85. The light source 14 may include asemiconductor laser and a drive circuit that drives this semiconductorlaser as a transmitting circuit, and the transmitting circuit mayinclude the optical amplification circuit 15. A structure of the opticalfrequency modulation multiplexing circuit 33 is shown in FIG. 13. InFIG. 13, the optical frequency modulation multiplexing circuit 33comprises an optical frequency modulation portion 22, an opticalfrequency local oscillation portion 32, and an optical multiplexer 23.

In FIG. 12, when multichannel AM video signals or QAM video signals thathave undergone frequency multiplication so as to have a frequency rangeof about 90 MHz to about 750 MHz as shown in FIG. 2A are input to theoptical signal transmitter 10, the signals are distributed by thedistribution circuit 11 into N signal parts. FIG. 12 shows a case inwhich N=3. The output of the distribution circuit 11 is input to each ofthe N optical frequency modulation multiplexing circuits 33 as amodulated input, and is subjected to frequency modulation by the opticalfrequency modulation portion 22 shown in FIG. 13.

In the optical frequency modulation multiplexing circuit 33 shown inFIG. 13, the optical frequency modulation portion 22 outputs afrequency-modulated optical signal, and the optical frequency localoscillation portion 32 outputs an optical local oscillation signalhaving a frequency apart from the optical frequency of the opticalsignal output from the optical frequency modulation portion 22 by afrequency substantially equal to the intermediate frequency. Thefrequency-modulated optical signal and the output from the opticalfrequency local oscillation portion 32 are multiplexed by the opticalmultiplexer 23.

The multiplexed optical signals output from the three optical frequencymodulation multiplexing circuits 33 are multiplexed by the opticalmultiplexing circuit 34, are then subjected to heterodyne detection bythe optical detection circuit 35, and are turned into an electric signalhaving a frequency equal to a difference between the optical frequencyof the frequency-modulated optical signal emitted from the opticalfrequency modulation portion and the optical frequency of the opticallocal oscillation signal emitted from the optical frequency localoscillation portion. A photodiode can be used as the optical detectioncircuit 35. The output of the optical detection circuit 35 is a widebandfrequency-modulated electric signal as shown in FIG. 2B. Thisfrequency-modulated electric signal is converted into an optical signalsubjected to intensity modulation by the light source 14, is thenamplified to a predetermined optical level by the optical amplificationcircuit 15, and is transmitted to the optical transmission path 85. Asemiconductor laser, such as a DFB-LD, can be used as the light source.

Herein, the frequency deviations of the N optical frequency modulationmultiplexing circuits are set to be substantially equal to each other.Further, the phases of electric signals obtained by subjecting theoptical signals emitted from the N optical frequency modulationmultiplexing circuits 33 to heterodyne detection by the opticaldetection circuit 35 are set to be substantially equal to each other.Thereby, the electric signals detected by the optical detection circuit35 have a noise quantity expressed as the sum total of electric powers,i.e., as an electric-power addition and have a signal componentexpressed as the sum total of voltages, i.e., as a voltage addition. Forexample, the length of a transmission path, such as an optical fiber,can be adjusted, or a phase adjuster can be used, in order to set themso that the phase of each output becomes mutually identical.

From this fact, it is understood that, when use is made of an opticalsignal transmitter that uses N optical frequency modulation multiplexingcircuits, the signal power becomes 20log(N), however, the noise powerbecomes 10log(N), and hence the signal-to-noise power in the output ofthe optical multiplexing circuit is improved by 10log(N)[dB] incomparison with a case in which use is made of an optical signaltransmitter that uses only one optical frequency modulation multiplexingcircuit.

With regard to distortions, the N optical frequency modulation portionsare different from each other in distortion characteristics, and, ifthey have distortion characteristics opposite in direction, offsettingcan be achieved in proportion to opposite distortions by a wavecombination, and hence the distortions can be reduced.

If the optical signal transmitter 10 of FIG. 12, instead of the opticaltransmitter 80, is applied to the optical signal transmission system inFIG. 1, the minimum light-receiving electric power of the optical signalreceiver can be reduced, and the transmission distance can belengthened, and the optical branching ratio can be enlarged between theoptical signal transmitter and the optical signal receiver.Additionally, if low distortion characteristics can be realized by theoptical signal transmitter, the quality of receiving video signals canbe improved.

Although the signal of FIG. 2A is used as an example of a signal to beinput to the optical signal transmitter in this embodiment, theinvention is not limited to this signal form.

Next, a ninth embodiment of the present invention is an optical signaltransmitter that uses N differential optical frequency modulationmultiplexing circuits to which electric signals distributed by adistribution circuit are input while being modulated, and is an opticalsignal transmission system using this optical signal transmitter. Thisembodiment of the present invention is shown in FIG. 14. FIG. 14 shows acase in which N=3. In FIG. 14, the optical signal transmitter 10comprises a distribution circuit 11, a differential optical frequencymodulation multiplexing circuit 36, an optical multiplexing circuit 34,an optical detection circuit 35, alight source 14 serving as atransmitting circuit, an optical amplification circuit 15, and anoptical transmission path 85. The light source 14 may include asemiconductor laser and a drive circuit that drives this semiconductorlaser as a transmitting circuit, and the transmitting circuit mayinclude the optical amplification circuit 15. A structure of thedifferential optical frequency modulation multiplexing circuit 36 isshown in FIG. 15. In FIG. 15, the differential optical frequencymodulation multiplexing circuit 36 comprises a differential distributor21, an optical frequency modulation portion 22-1, an optical frequencymodulation portion 22-2, and an optical multiplexer 23.

In FIG. 14, when multichannel AM video signals or QAM video signals thathave undergone frequency multiplication so as to have a frequency rangeof about 90 MHz to about 750 MHz as shown in FIG. 2A are input to theoptical signal transmitter 10, the signals are distributed by thedistribution circuit 11 into N signal parts. The output of thedistribution circuit 11 is input to each of the N differential opticalfrequency modulation multiplexing circuits 36 as a modulated input.

In the differential optical frequency modulation multiplexing circuit 36shown in FIG. 15, the output from the distribution circuit 11 isdistributed by the differential distributor 21 into two electric signalsin which phases have been inverted. The two electric signals are turnedinto frequency-modulated optical signals in the optical frequencymodulation portion 22-1 and the optical frequency modulation portion22-2, respectively. The optical frequency of an optical signal outputfrom the optical frequency modulation portion 22-1 and the opticalcenter frequency of an optical signal output from the optical frequencymodulation portion 22-2 are apart from each other by the intermediatefrequency. The frequency-modulated optical signals emitted from theoptical frequency modulation portion 22-1 and the optical frequencymodulation portion 22-2 are multiplexed by the optical multiplexer 23,and are output to the optical multiplexing circuit 34 shown in FIG. 14.Herein, the intermediate frequencies in the N differential opticalfrequency modulation multiplexing circuits 36 are set to besubstantially equal to each other.

The optical signals output from the N differential optical frequencymodulation multiplexing circuits 36 are multiplexed by the opticalmultiplexing circuit 34, are then subjected to heterodyne detection bythe optical detection circuit 35, and are turned into an electric signalhaving a frequency equal to a difference between the optical frequencyof the frequency-modulated optical signal emitted from the opticalfrequency modulation portion 22-1 and the optical frequency of thefrequency-modulated optical signal emitted from the optical frequencymodulation portion 22-2. A photodiode can be used as the opticaldetection circuit 35. The output of the optical detection circuit 35 isa wideband frequency-modulated electric signal as shown in FIG. 2B. Thisfrequency-modulated electric signal is converted into an optical signalsubjected to intensity modulation by the light source 14, is thenamplified to a predetermined optical level by the optical amplificationcircuit 15, and is transmitted to the optical transmission path 85. Asemiconductor laser, such as a DFB-LD, can be used as the light source.

Herein, the frequency deviations of the N differential optical frequencymodulation multiplexing circuits are set to be substantially equal toeach other. Further, the phases of electric signals obtained bysubjecting the optical signals emitted from the N differential opticalfrequency modulation multiplexing circuits 36 to heterodyne detection bythe optical detection circuit 35 are set to be substantially equal toeach other. Thereby, the electric signals detected by the opticaldetection circuit 35 have a noise quantity expressed as the sum total ofelectric powers, i.e., as an electric-power addition and have a signalcomponent expressed as the sum total of voltages, i.e., as a voltageaddition. For example, the length of a transmission path, such as anoptical fiber, can be adjusted, or a phase adjuster can be used, inorder to set them so that the phase of each output becomes mutuallyidentical.

From this fact, it is understood that, when use is made of an opticalsignal transmitter that uses N differential optical frequency modulationmultiplexing circuits, the signal power becomes 20log(N), however, thenoise power becomes 10log(N), and hence the signal-to-noise power in theoutput of the optical multiplexing circuit is improved by 10log(N)[dB].

With regard to distortions, the 2N optical frequency modulation portionsare different from each other in distortion characteristics, and, ifthey have distortion characteristics opposite in direction, offsettingcan be achieved in proportion to opposite distortions by a wavecombination, and hence the distortions can be reduced.

If the optical signal transmitter 10 of FIG. 14, instead of the opticaltransmitter 80, is applied to the optical signal transmission system inFIG. 1, the minimum light-receiving electric power of the optical signalreceiver can be reduced, and the transmission distance can belengthened, and the optical branching ratio can be enlarged between theoptical signal transmitter and the optical signal receiver.Additionally, if low distortion characteristics can be realized by theoptical signal transmitter, the quality of receiving video signals canbe improved.

Although the signal of FIG. 2A is used as an example of a signal to beinput to the optical signal transmitter in this embodiment, theinvention is not limited to this signal form.

Additionally, the optical transmitter and the optical transmissionsystem of the present invention can be used in a case in which thenetwork of the optical transmission path is a passive double startopology (PDS topology), as well as a single star topology (SStopology).

1. An optical signal transmitter for applying frequency modulation toamplitude-modulated electric signals that have undergone frequencydivision multiplexing to optically transmit the electric signals, theoptical signal transmitter comprising: a distribution circuit fordistributing the electric signals into a plurality of signal parts andoutputting the signal parts; a plurality of frequency modulation meansfor applying frequency modulation to each output of the distributioncircuit and emitting each output, the plurality of frequency modulationmeans being substantially equal to each other in frequency deviation andin intermediate frequency and being substantially identical in the phaseof each output; a multiplexing means for multiplexing outputs of theplurality of frequency modulation means and outputting multiplexedoutputs; and a transmitting circuit for outputting optical signalssubjected to intensity modulation by the output of the multiplexingmeans to an optical transmission path.
 2. An optical signal transmitteraccording to claim 1, wherein the distribution circuit distributes andoutputs the electric signals into N signal parts (N is an integer whichis two or greater), the plurality of frequency modulation means are N FMbatch conversion circuits from which each output of the distributioncircuit is subjected to frequency modulation to output, and themultiplexing means is an optical multiplexing circuit for multiplexingoutputs of the N FM batch conversion circuits and outputting multiplexedoutputs, wherein the N FM batch conversion circuits are set to besubstantially equal to each other in frequency deviation and inintermediate frequency and to be substantially identical in the phase ofeach output.
 3. An optical signal transmitter according to claim 2,wherein each of the FM batch conversion circuits comprises: an opticalfrequency modulation portion for outputting a frequency-modulatedoptical signal applied frequency modulation to the electric signalsinput from the distribution circuit as a modulated input; an opticalfrequency local oscillation portion for outputting an optical localoscillation signal having a frequency apart from an optical centerfrequency of the frequency-modulated optical signal output from theoptical frequency modulation portion by a frequency substantially equalto an intermediate frequency; an optical multiplexer for multiplexingthe frequency-modulated optical signal and the optical local oscillationsignal together and outputting a multiplexed optical signal; and anoptical detector for applying heterodyne detection to the multiplexedoptical signal output from the optical multiplexer and outputting anelectric signal having a frequency equal to a difference between anoptical frequency of the frequency-modulated optical signal and anoptical frequency of the optical local oscillation signal.
 4. An opticalsignal transmitter according to claim 2, wherein each of the FM batchconversion circuits comprises: a differential distributor fordistributing the electric signal output from the distribution circuitinto two electric signals in which phases have been inverted; a firstoptical frequency modulation portion for outputting a firstfrequency-modulated optical signal applied frequency modulation to oneof the two electric signals input from the differential distributor as amodulated input; a second optical frequency modulation portion foroutputting a second frequency-modulated optical signal applied frequencymodulation to the other one of the two electric signals input form thedifferential distributor as a modulated signal, the secondfrequency-modulated optical signal having an optical frequency apartfrom an optical center frequency of the first frequency-modulatedoptical signal by a frequency substantially equal to an intermediatefrequency; an optical multiplexer for multiplexing the firstfrequency-modulated optical signal and the second frequency-modulatedoptical signal and outputting a multiplexed optical signal; and anoptical detector for applying heterodyne detection to the multiplexedoptical signal output from the optical multiplexer and outputting anelectric signal having a frequency equal to a difference between anoptical frequency of the first frequency-modulated optical signal and anoptical frequency of the second frequency-modulated optical signal. 5.An optical signal transmitter according to claim 2, wherein each of theFM batch conversion circuits comprises a voltage-controlled oscillatorfor converting the electric signal output from the distribution circuitinto a signal having a frequency corresponding to a voltage thereof withan intermediate frequency as a center frequency and outputting aconverted signal.
 6. An optical signal transmitter according to claim 2,wherein each of the FM batch conversion circuits comprises: adifferential distributor for distributing the electric signal outputfrom the distribution circuit into two electric signals in which phaseshave been inverted; a first voltage-controlled oscillator for outputtinga first frequency-modulated optical signal obtained by converting one ofthe two electric signals emitted from the differential distributor so asto have a frequency corresponding to a voltage thereof; a secondvoltage-controlled oscillator for outputting a secondfrequency-modulated optical signal obtained by converting the other oneof the two electric signals emitted from the differential distributor soas to have a frequency corresponding to a voltage thereof, a centerfrequency of the second frequency-modulated optical signal being apartfrom a center frequency of the first frequency-modulated signal by afrequency substantially equal to an intermediate frequency; a mixer formixing the first frequency-modulated signal output from the firstvoltage-controlled oscillator and the second frequency-modulated signaloutput from the second voltage-controlled oscillator together; and alow-pass filter for allowing the electric signal output from the mixerto pass through, the electric signal having a frequency equal to adifference between a frequency of the first frequency-modulated signaland a frequency of the second frequency-modulated signal.
 7. An opticalsignal transmitter according to claim 1, wherein the distributioncircuit distributes the electric signal into two signals (N=2) andoutputs these signals, the frequency modulation means includes: a firstoptical frequency modulation portion for outputting a firstfrequency-modulated optical signal applied frequency modulation to oneof the two electric signals input from the distribution circuit as amodulated input; a first optical frequency local oscillation portion foroutputting a first optical local oscillation signal having an opticalfrequency apart from an optical center frequency of the firstfrequency-modulated optical signal output from the first opticalfrequency modulation portion by a frequency substantially equal to anintermediate frequency; a first optical multiplexer for multiplexing thefirst frequency-modulated optical signal and the first optical localoscillation signal so as to become identical in polarization directionand outputting a first multiplexed optical signal; a second opticalfrequency modulation portion for outputting a second frequency-modulatedoptical signal applied frequency modulation to the other one of the twoelectric signals input from the distribution circuit as a modulatedinput; a second optical frequency local oscillation portion foroutputting a second optical local oscillation signal having an opticalfrequency apart from an optical center frequency of the secondfrequency-modulated optical signal output from the second opticalfrequency modulation portion by a frequency substantially equal to theintermediate frequency; and a second optical multiplexer formultiplexing the second frequency-modulated optical signal and thesecond optical local oscillation signal together so as to becomeidentical in polarization direction and outputting a second multiplexedoptical signal, the multiplexing means is a third optical multiplexerfor multiplexing the first multiplexed optical signal output from thefirst optical multiplexer and the second multiplexed optical signaloutput from the second optical multiplexer so that a polarizationdirection of the first multiplexed optical signal becomes perpendicularto a polarization direction of the second multiplexed optical signal andoutputting a third muptiplexed optical signal, the optical signaltransmitter further comprising: an optical detector for applyingheterodyne detection to the third multiplexed optical signal output fromthe third optical multiplexer and outputting an electric signal having afrequency equal to a difference between an optical frequency of thefirst frequency-modulated optical signal and an optical frequency of thefirst optical local oscillation signal and an electric signal having afrequency equal to a difference between an optical frequency of thesecond frequency-modulated optical signal and an optical frequency ofthe second optical local oscillation signal; and a transmitting circuitfor outputting an optical signal subjected to intensity modulation by anoutput of the optical detector to an optical transmission path; whereinthe first optical frequency modulation portion and the second opticalfrequency modulation portion are set to be substantially equal to eachother in frequency deviation, wherein a phase of an electric signalobtained by subjecting the first multiplexed optical signal toheterodyne detection in the optical detector is set to be substantiallyidentical to a phase of an electric signal obtained by subjecting thesecond multiplexed optical signal to heterodyne detection in the opticaldetector.
 8. An optical signal transmitter according to claim 1, whereinthe distribution circuit distributes the electric signal into twoelectric signals (N=2) and outputs these electric signals, the frequencymodulation means includes: a first differential distributor fordistributing one of the two electric signals output from thedistribution circuit into two electric signals in which phases have beeninverted; a first optical frequency modulation portion for outputting afirst frequency-modulated optical signal applied frequency modulation toone output of the first differential distributor as a modulated input; asecond optical frequency modulation portion for receiving the otheroutput of the first differential distributor as a modulated input andoutputting a second frequency-modulated optical signal, the secondfrequency-modulated optical signal having an optical frequency apartfrom an optical center frequency of the first frequency-modulatedoptical signal output from the first optical frequency modulationportion by a frequency substantially equal to an intermediate frequency;a first optical multiplexer for multiplexing the firstfrequency-modulated optical signal and the second frequency-modulatedoptical signal together so as to become identical in polarizationdirection and outputting a first multiplexed optical signal; a seconddifferential distributor for distributing the other output of thedistribution circuit into two electric signals in which phases have beeninverted; a third optical frequency modulation portion for outputting athird frequency-modulated optical signal receiving one output of thesecond differential distributor as a modulated input to apply frequencymodulation to the one output of the second differential distributor; afourth optical frequency modulation portion for receiving the other oneof the two electric signals output from the second differentialdistributor as a modulated input and outputting a fourthfrequency-modulated optical signal, the fourth frequency-modulatedoptical signal having an optical frequency apart from an optical centerfrequency of the third frequency-modulated optical signal output fromthe third optical frequency modulation portion by a frequencysubstantially equal to the intermediate frequency; and a second opticalmultiplexer for multiplexing the third frequency-modulated opticalsignal and the fourth frequency-modulated optical signal together so asto become identical in polarization direction and outputting a secondmultiplexed optical signal, the multiplexing means is a third opticalmultiplexing for multiplexing the first multiplexed optical signaloutput from the first optical multiplexer and the second multiplexedoptical signal output from the second optical multiplexer so that apolarization direction of the first multiplexed optical signal becomesperpendicular to a polarization direction of the second multiplexedoptical signal and outputting a third multiplexed optical signal, theoptical signal transmitter further comprising: an optical detector forapplying heterodyne detection to the third multiplexed optical signaloutput from the third optical multiplexer and outputting an electricsignal having a frequency equal to a difference between an opticalfrequency of the first frequency-modulated optical signal and an opticalfrequency of the second frequency-modulated optical signal and anelectric signal having a frequency equal to a difference between anoptical frequency of the third frequency-modulated optical signal and anoptical frequency of the fourth frequency-modulated optical signal; anda transmitting circuit for outputting an optical signal subjected tointensity modulation by an output of the optical detector to an opticaltransmission path; wherein the first frequency modulation portion,second frequency modulation portion, third frequency modulation portion,and fourth optical frequency modulation portion are set to besubstantially equal to each other in frequency deviation, and a phase ofan electric signal obtained by subjecting the first multiplexed opticalsignal to heterodyne detection in the optical detector is set to besubstantially identical to a phase of an electric signal obtained bysubjecting the second multiplexed optical signal to heterodyne detectionin the optical detector.
 9. An optical signal transmitter according toclaim 1, wherein the distribution circuit distributes the electricsignal into N signals (N is an integer which is two or greater) andoutputs these signals, the frequency modulation means includes N opticalfrequency modulation multiplexing circuits for multiplexing afrequency-modulated optical signal applied frequency modulation to eachoutput input from the distribution circuits as a modulated input and anoptical local oscillation signal having an optical frequency apart froman optical center frequency of the frequency-modulated optical signal bya frequency substantially equal to an intermediate frequency together,and outputting a multiplexed signal, the multiplexing means is anoptical multiplexing circuit for multiplexing outputs of the N opticalfrequency modulation multiplexing circuits and outputting a multiplexedsignal, the optical signal transmitter further comprising: an opticaldetection circuit for applying heterodyne detection to an output of theoptical multiplexing circuit and outputting an electric signal having afrequency equal to a difference between an optical frequency of thefrequency-modulated optical signal and an optical frequency of theoptical local oscillation signal; and a transmitting circuit foroutputting an optical signal subjected to intensity modulation by anoutput of the optical detection circuit to an optical transmission path;wherein the N optical frequency modulation multiplexing circuits are setto be substantially equal to each other in frequency deviation and to besubstantially identical to each other in intermediate frequency and inthe phase of each of the N superposed electric signals obtained byapplying heterodyne detection to multiplexed optical signals output fromthe N optical frequency modulation multiplexing circuits in the opticaldetection circuit.
 10. An optical signal transmitter according to claim1, wherein the distribution circuit distributes the electric signal intoN signals (N is an integer which is two or greater) and outputs thesesignals, the frequency modulation means includes N differential opticalfrequency modulation multiplexing circuits for distributing each outputof the distribution circuit into two electric signals in which phaseshave been inverted, multiplexing a first frequency-modulated opticalsignal applied frequency modulation to one of the two electric signalsinput from the distribution circuit as a modulated input and a secondfrequency-modulated optical signal having an optical frequency apartfrom an optical center frequency of the first frequency-modulatedoptical signal by a frequency substantially equal to an intermediatefrequency, the second frequency-modulated optical signal appliedfrequency modulation to the other one of the two electric signals inputfrom the distribution circuit as a modulated input, and outputting amultiplexed signal, the multiplexing means is an optical multiplexingcircuit for multiplexing outputs of the N differential optical frequencymodulation multiplexing circuits and outputting a multiplexed signal,the optical signal transmitter further comprising: an optical detectioncircuit for applying heterodyne detection to an output of the opticalmultiplexing circuit and outputting an electric signal having afrequency equal to a difference between an optical frequency of thefirst frequency-modulated optical signal and an optical frequency of thesecond frequency-modulated optical signal; and a transmitting circuitfor outputting an optical signal subjected to intensity modulation by anoutput of the optical detection circuit to an optical transmission path;wherein the N differential optical frequency modulation multiplexingcircuits are set to be substantially equal to each other in frequencydeviation and to be substantially identical to each other inintermediate frequency and in the phase of each of the N superposedelectric signals obtained by applying heterodyne detection tomultiplexed optical signals output from the N differential opticalfrequency modulation multiplexing circuits in the optical detectioncircuit.
 11. An optical signal transmission system comprising: theoptical signal transmitter according to any one of claims 1 to 10, andan optical signal receiver including a photoelectric conversion meansconnected to the optical signal transmitter via an optical transmissionpath and a frequency demodulation means for demodulating an output ofthe photoelectric conversion means.